1. Field of the Invention
The present inention relates to a focus condition detecting device in which an image of a target object formed by an objective lens is re-formed, by first and second refocusing lenses disposed symmetrically with respect to the optical axis of the objective lens, into first and second optical images on first and second detection means for detecting the first and second optical images, respectively and a distance between the first and second optical images is calculated on the basis of illuminance distributions of the first and second optical images detected by the first and second detection means, respectively such that focus condition of the objective lens relative to the target object, and especially the amount of deviation (defocus amount) of the image of the target object from a predetermined image forming plane is detected from the distance between the first and second optical images.
2. Description of the Prior Art
FIGS. 1 and 2 show an optical system of a prior art focus condition detecting device of this kind and formation of images therein, respectively. The known optical system includes an objective lens 2, a predetermined focal plane 4 positioned rearwardly of the objective lens 2 and a condenser lens 6 positioned rearwardly of the predetermined focal plane 4. The condenser lens 6 is constituted by a spherical lens. Futhermore, the known optical system includes a pair of refocusing lenses 8 and 10 positioned rearwardly of the condenser lens 6 and a pair of line sensors 12 and 14 positioned at image forming planes of the refocusing lenses 8 and 10, respectively. Each of the line sensors 12 and 14 has a charge coupled device (CCD) used as a photo-sensor array. First and second images of a target object are, respectively, formed on the line sensors 12 and 14. These images come close to an optical axis 18 in a front focus condition in which an image of a target object to be focused is formed forwardly of the predetermined focal plane 4, as indicated by arrows A, a, and a' in FIG. 2. On the contrary, the images are spaced away from the optical axis 18 in a rear focus condition in which the image of the target object is formed rearwardly of the predetermined focal plane 4, as indioated by arrow B, b and b' in FIG. 2. In an in-focus condition in which the image of the target object is formed on the predetermined focal plane 4, a distance between corresponding points of the two images is set to a specific distance determined by design conditions of the optical system. Therefore, in principle, the focus condition of the optical system can be detected by measuring the distance between the two images.
The detection of the distance mentioned above can be made as follows.
Now, assume that the first and second image sensors 12 and 14 are comprised of ten and sixteen cells of photodiodes (a.sub.1, . . . ,a.sub.10) and (b.sub.1, . . . ,b.sub.16), respectively. Considering now sets each of which is comprised of ten successive cells included in the second image sensor, seven sets B.sub.1, B.sub.2, . . . ,B.sub.7 can be obtained. The focus condition can be sought by calculating individual correlation relation between the image received by ten cells of the first image sensor 12 and the image received by each of seven sets of the second image sensor 14.
Namely, correlation calculations are made with use of correlation functions: ##EQU1## (i=1,2, . . . 7)
For example, if the image detected by the first sensor 12 coincides with the image detected by the first set B.sub.1 of the second image sensor 14, the correlation function S.sub.1 becomes minimum among seven correlation functions S.sub.1, S.sub.2, . . . ,S.sub.7. When either one set of the second sensor is found which gives the minimum value with respect to these correlation functions, the distance between two images is determined from the number of the set having been found minimum and a focus condition is detected based on the distance determined. These calculations are carried out by a correlation calculation means 16.
In an automatic focus adjusting device of a camera having such an optical system for detecting a focus condition as mentioned above, there is provided a control circuit inoluding at least one micro-computer and a sequence comprising integration (charge accumulation) operations by respective CCD image sensors for detecting light intensity distributions of an object: a calculation for detecting a focus condition according to outputs from the CCD image sensors (a calculation of an amount and direction of defocus), a lens drive based on the defocus amount and direction detected: a stop of an objective lens at an in-focus position and a shutter release (if a shutter button of the camera is pushed down) is controlled by the control circuit acoording to programs stored therein. This auto-focus adjusting device repeats the sequential auto-focus control even when the objective lens is approached to an in-focus position detected in order to locate the objective lens thereat. Such a focus condition detection is performed with respect to a portion of an object image located within a limited area in the field of view of the objective lens of the camera (a focus detecting area).
Let's consider such a case, wherein a picture of a moving object is taken with the use of a camera having an auto-focus adjusting device of the above described type which is sensitive only to a portion of the object located within the limited area in the field of view of an objective lens.
In such a case, a distance from the object to the camera is varied according to the movement thereof. Therefore, it becomes necessary to repeat a focus condition detection operation and a lens driving operation for the focus adjustment based upon the result of the focus condition detection operation. In other words, a continuous auto-focus adjusting mode (hereinafter, referred to a "continuous AF mode") is needed in which the focus condition detection operation and the lens drive operation are repeated even after an in-focus condition has been obtained once. Further, it becomes necessary to pursue a main target so that it oan be located within the limited focus detection area during autofocusing in the continuous auto-focus mode, and, therefore, it is desirable to enlarge the focus detection area as large as possible. However, in the case wherein a wide focus detection area is set, the result of a focus condition detection is greatly affected by a secondary target having a high contrast as far as the image thereof is located within the wide focus detection area.
In order to avoid such a disadvantage as mentioned just above, there has been proposed a focus condition detecting system wherein a CCD line image sensor as shown in FIG. 3 is employed (See U.S. Pat. No. 4,636,624). This line image sensor 15 has a first and second portions referred to as a standard and reference portion L and R, respectively. In the standard portion L, three overlapped blocks I, II and III for detecting a focus condition are defined so as to have elements from l.sub.1 to l.sub.20, elements from l.sub.11 to l.sub.30 and elements from l.sub.21 to l.sub.40, respectively.
ln this system, three correlation calculations are executed with use of these first to third blocks (I), (II), (III) and the reference portion R. A distance between two images refocused on the standard and reference portions is calculated with the use of a block which gives the highest correlation among the first to third blocks. A defocus amount and direction is calculated based on the image distance obtained and the objective lens is driven corresponding to the defocus amount and direction.
However, in this system, it takes a relatively long time to execute the focus condition detection operation since the correlation calculation must be repeated three times. Accordingly, this system is not so effective for a moving object although it is suitable for pursuing a moving object.